This invention relates to a mixing apparatus and method in general, and, in particular, to a mixing impeller with the blades having two different surface configurations and a method using such an apparatus.
In many areas of technology, it is frequently desirable to mix a fluid with one or more other substances. It has been necessary to mix gasses, liquids, and solids, of a powdered or particulate nature, together with a liquid contained in a tank. The mixing process and mixer performance involve the interrelation of three known factors: an impeller, a fluid, and a mixing vessel which may include baffles that contain the fluid. The impeller is used to move and agitate the fluid, and mixing results from this fluid motion.
Impellers may be divided into two general classifications, axial and radial flow, depending upon their shape, positioning, and vessel geometry. The flow pattern developed by a mixer will depend upon the geometric configuration of the impeller and the physical properties of the fluid mixture. For examples of various shaped impellers, including a propeller, an arrowhead impeller, and a flat blade impeller, see U.S. Pat. Nos. 2,165,916; 2,384,952; and 2,637,583, respectively.
In a gas-liquid mixing operation, the principal function of the impeller is to disperse a gas stream to an effective bubble size and interfacial area. At the same time, the impeller should increase the turbulence in the liquid and provide as uniform as possible distribution of the gas within the liquid. The flat blade turbine has proven to be particularly useful in achieving the desired gas-liquid mixing.
The flat blade turbine impeller has a disc which is open on both sides in order to assure good contact for any gas both above and below the turbine level. Hence, it provides good suction above the hub of the disc to draw bubbles downward to the impeller and prevents short-circuiting of gas from below the impeller upward along the impeller shaft. Accordingly, the flat blade turbine imparts both an axial flow and a radial flow, thereby maximizing the turbulence of the liquid with which the gas is mixed.
The blades of such a turbine are spaced radially from the hub since it has been found by experiment that little fluid motion takes place adjacent to the hub of the impeller. In additon, its blades are not shrouded (covered) thereby further contributing to their ability to create turbulence as the fluid is subjected to both axial and radial forces generated by the turbine. Such characteristics of impeller design (radial spaced blades, uncovered, and unshrouded blades), served to distinguish mixing impellers from impellers used in other types of devices, such as pumps.
In pumps, where turbulence is undesirable, it is a common practice to provide flow in only one direction (axial or radial). Usually only one surface of a disc carries blades, and the blades of the pump impeller normally extend from a central hub to the end of the disc in order to separate and seal a plurality of pump chambers. For an example of one such type of pumping impeller, see U.S. Pat. No. 3,136,254. Because of their marked differences in purpose and function, pump impellers are not generally considered to be useful for mixing operations and vice versa.
During some mixing operations, it is often desirable to vary the power input to a mixer motor if the physical properties of the mixture change, e.g. the mixture's viscosity increases. Then the load on the motor may increase, and the motor speed will have to be adjusted in order to prevent an overload. In other operations it is desirable to operate the mixing process at a different rate. For example, in a fermentation cycle, oxygen is supplied to propagate micro-organisms. At the beginning of such a cycle, the population is minimal and the oxygen demand is low. Hence, a relatively low gas rate and a low mixer speed (low horsepower) is sufficient during the start of such a cycle. However, a higher gas rate and higher mixer speed is needed as the population grows.
In such operations, it has in the past been necessary to provide a variable speed control for the motor or a special two-speed motor and appropriate switchgear in order to accomplish the aforementioned desired result. Such variable speed controls and special motors are expensive. Accordingly, it would be desirable to have an inexpensive means for altering the power required to turn an impeller in a mixing apparatus without adjusting the speed of the motor.
In other mixing operations, especially related to mass transfer, such as the fermentation cycle, the efficiency of the mixing impeller is a function of mass transferred per unit power consumed. Accordingly, it would be desirable to provide a more energy efficient impeller for mass transfer operations.